This study will address the physiology of leptin secretion in lean and obese subjects and will investigate the potential role of leptin as a mediator of the neuroendocrine response to food deprivation in humans. Data derived from these studies not only will increase understanding of the normal physiologic of leptin's secretion and its role in the neuroendocrine response to fasting, but will also provide important insights into the neuroendocrine response to therapeutic dieting in obesity. Thus, these studies may provide the basis for future therapeutic interventions for obesity, a major public health problem.